Major Concepts in Physics Lecture 22.

1. Major Concepts in Physics Lecture 22. Prof Simon Catterall Office 309 Physics, x 5978 smc@physics.syr.edu http://physics/courses/PHY102.08Spring PHY102

2. The nucleus - recap • Atom composed of small, dense core of protons (charge +1) and neutrons (charge 0) surrounded by cloud of electrons (charge -1) • Nucleus held together by strong nuclear force (QCD) • Nuclear energy scales MeV – characterizes energies of allowed levels and binding energy PHY102

3. PHY102

4. Fusion • Most stable (largest binding energy) atomic nucleus is iron A=58, Z=26 • Smaller nuclei can fuse together to make a more stable nucleus eg in Sun 2H+3H  4He+n+17.6MeV • This process cannot occur at room temperature since H must get very close together to overcome electrical repulsion and strong nuclear force to take over PHY102

5. Fusion II • In general this process of fusing two smaller nuclei to make a larger one continues to generate energy up to iron • Once most of the stars core is iron cannot liberate any more energy this way – no outward radiation pressure to keep star in equilibrium – collapses under gravity • Supernova or maybe black hole produced PHY102

6. Fusion III • Fusion offers the possibility of huge amounts of power without the radioactive by products of fission • Also very stable – most accidents with fusion reactor will lower its temperature and switch of the fusion reactions • But very difficult to achieve. Need to sustain temps of 107 K! PHY102

7. Fig. 29.18 PHY102

8. Fission • Can also increase stability of stable nucleus by splitting it into two or more pieces – fission • Eg. Uranium-235 (i.e A=235) can spontaneously decay eg 235U (+n)90Rb+143Cs+2n+200 MeV! • Note: more neutrons out than in – possibility of chain reaction PHY102

9. Nuclear reactors • U-235 occurs naturally as an isotope of the more common (and stable) U-238. • Concentrate. And irradiate with (slow) neutrons  fission. Emitted neutrons too fast to cause more fissions – slow using a moderator (eg heavy water, graphite). • By controlling number and energy of neutrons in reactor can control number of fissions per second and produce a steady energy output • Problems – radioactive fission products, stability PHY102

10. Fig. 29.16 PHY102

11. Radioactive decay • Unstable nuclei can and often do undergo less drastic changes to increase stability radioactive decay • Three types: • Alpha decay – emit He nucleus • Beta decay – emit a high energy electron • Gamma decay – emit a high energy photon PHY102

12. Demo – alpha and beta emitters PHY102

13. Alpha decay • Example: 238U  234Th+4He • Note: alpha’s can be stopped using a sheet of cardboard • Mass of products is less than mass of original nucleus. The extra negative binding energy is seen as the energy released – here 4 MeV or so. PHY102

14. Fig. 29.4 PHY102

15. Beta decay • In beta decay ene electron is emitted and a neutron in the nucleus is converted into a proton • Process involves the weak nuclear force • Example: 137Cs (Z=55)  137Ba(Z=56)+e+energy • Stopped by a thin sheet of metal PHY102

16. Gamma decay • Neutron/proton in excited state drops to lower energy state by emitting high energy photon. • No change in Z or A • Can penetrate a meter or so of concrete. PHY102

17. Half-life • Radioactive decays are characterized by their half-life: • Time required for half atoms in some sample to decay T1/2 • Can show T1/2=0.693/(probability to decay per sec) • Number remaining after time t given by N(t)=N0(1/2)t/T PHY102

18. Fig. 29.8 PHY102

19. Problem • Half-life of 13N is 9.965 mins. If a sample contains 3.2x1012 atoms at t=0 how many remain 40 min later ? • Use formula: • Ans= 1.98x1011 atoms PHY102

20. History of elementary components of matter • Discovery of atoms as building blocks of macroscopic objects circa 1800 – chemistry as different ways to add 100 different basic atoms together • Circa 1900 scientists learnt that atoms were not elementary – built from electrons, protons and neutrons. All of chemistry, nuclear and atomic physics could be understood in terms of these ingredients PHY102

21. More elementary particles … • In high energy colliders 1950-70 many more particles were produced eg pions, kaons, sigmas, deltas etc (hadrons) • Signal that all these particles were built out of simpler ingredients ? • Like all different atoms built from e,p,n …. • Yes ….! PHY102

22. Particle Physics • Now know that protons, neutrons are not truly fundamental • Composed of things called quarks • Quarks come in 2 flavors – called up and down • Proton is (uud) and neutron (ddu) • Electric charge u=2/3 electron charge • Electric charge d=-1/3 electron charge PHY102

23. Fig. 30.1 PHY102

24. Colors too • Actually the force that binds these quarks into hadrons QCD requires quarks to have another property called color=(red,green,blue) • Fundamental property of QCD that hadrons are colorless • What about electrons ? Example of elementary particle called lepton . Two flavors needed – electron and neutrino PHY102

25. Summary • Most (!) matter and most of the phenomena we see around us understood in terms of • u,d quarks and (e,n) • QCD binds quarks into p,n,hadrons • Weak nuclear force (beta decay ud) • EM forces • Gravity • Detailed description given by quantum mechanics and relativity PHY102